Interpretive Summary: Winter wheat phenology varies among shoots on the plant to main stems on plants within a plot to locations across a landscape. Rarely have phenological measurements addressed variation across a landscape with varying soils and topography. Many models exist to predict wheat phenology for small treatment plots based on the primary drivers of temperature and photoperiod, with secondary factors of water deficits, nutrient availability, and atmospheric CO2 levels occasionally considered. When considering winter wheat phenology across a landscape with varying topography and soils, differences in phenology would be expected as the drivers vary. We conducted a 6-year experiment measuring winter wheat phenology across approximately a 100-ha landscape with topographic and soil variation. Air and soil temperature and soil water content were measured at each location. Preliminary analysis on 3 of the 6 years of data shows that depending on location, the time of main stem jointing, flag leaf completing growth, heading, anthesis, and physiological maturity varied as much as 23, 17, 15, 18, and 9 days, respectively, within a year. The location rankings often did not remain consistent across growth stages. Maturity growth stage had the lowest range of variation, and in our environments may reflect that regardless of location and year, resources universally across locations became sufficiently limiting at nearly the same time causing growth to cease. Given that the same cultivar was used, what are the sources of this variation? One important source of variation was planting date that greatly impacted seedling emergence patterns. In the one year with two planting dates, the variation decreased by 17 days for the jointing growth stage within a planting date, and often for all stages other than maturity, typically 10 or more days. Variation due to different air and soil temperatures will be estimated by conversion to thermal time. In addition, the influence of soil water content on thermal time will be considered.

Technical Abstract:
Winter wheat phenology varies among shoots on the plant to main stems on plants within a plot to locations across a landscape. Most often phenological measurements have focused on small treatment plots under presumably similar soils and topography. Many models exist to predict wheat phenology for small treatment plots based on the primary drivers of temperature and photoperiod, with secondary factors of water deficits, nutrient availability, and atmospheric CO2 levels occasionally considered. When considering winter wheat phenology across a landscape with varying topography and soils, differences in phenology would be expected as the drivers vary. We conducted a 6-year experiment measuring winter wheat phenology across approximately a 100-ha landscape with topographic and soil variation. At each of 10 landscape positions per growing season, wheat emergence and biomass at different growth stages were sampled in randomly nested patterns. Air and soil temperature and soil water content were measured at each location. Preliminary analysis on 3 of the 6 years of data shows that depending on location, the time of main stem jointing, flag leaf completing growth, heading, anthesis, and physiological maturity varied as much as 23, 17, 15, 18, and 9 days, respectively, within a year. The location rankings often did not remain consistent across growth stages. Maturity growth stage had the lowest range of variation, and in our environments may reflect that regardless of location and year, resources universally across locations became sufficiently limiting at nearly the same time causing growth to cease. Given that the same cultivar was used, what are the sources of this variation? The photoperiod factor can be eliminated from consideration since it was similar at all locations. One important source of variation was planting date that greatly impacted seedling emergence patterns. In the one year with two planting dates, the variation decreased by 17 days for the jointing growth stage within a planting date, and often for all stages other than maturity, typically 10 or more days. Variation due to different air and soil temperatures will be estimated by conversion to thermal time. In addition, the influence of soil water content on thermal time will be considered.